Field of the Invention
The present invention relates to a water heating apparatus and system, and more particularly to a water heating apparatus and system having a scale detecting function.
Description of the Background Art
Use of a water heating apparatus for a long time causes a scale to adhere in a pipe of a heat exchanger. In particular, in the case where so-called hard water containing a large quantity of calcium ion and magnesium ion is used, the amount of adhesion of the scale becomes greater. When use of the water heating apparatus with adhesion of the scale continues, normal heat transmission of the heat exchanger may be impaired by the scale, thus damage such as cracks in the heat exchanger may occur due to generation of thermal stress caused by the scale. Accordingly, it is necessary to detect adhesion of the scale appropriately. Japanese Patent Laying-Open No. 2008-138952 and Japanese Patent Laying-Open No. 2014-47980 disclose a method of detecting scale in a water heating apparatus.
Japanese Patent Laying-Open No. 2008-138952 describes that adhesion of scale is determined based on whether a post-boiling temperature after heating is stopped exceeds a threshold value. Furthermore, Japanese Patent Laying-Open No. 2014-47980 describes that adhesion of scale is determined based on comparing a heat exchanger's heat exchanging efficiency with a threshold value. Japanese Patent Laying-Open No. 2014-47980 provides one threshold value for determination, whereas Japanese Patent Laying-Open No. 2008-138952 employs different threshold values. However, the temperature monitored in Japanese Patent Laying-Open No. 2008-138952 is a post-boiling temperature detected after heating is stopped, rather than temperature detected while a combustion operation is implemented. Accordingly, during combustion, adhesion of scale cannot be determined or an error cannot be output based on the result of such determination. Accordingly, damage such as cracking of the heat exchanger which may arise during combustion cannot be prevented.